In recent years, a Redundant Arrays of Inexpensive Disks (RAID) mechanism has frequently been adopted in a data storage apparatus for storing video data to improve reliability and availability of contained memory units and realize faster access (for example, see Jpn. Pat. Appln. KOKAI Publication No. 11-96081). Various RAID levels are available. RAID 5 is widely adopted for its excellent balance of reliability/availability and economic efficiency. RAID 5 distributes parity information to memory units. RAID 5 can compensate data upon failure of one memory unit, realize faster reading at normal condition by striping, and provide economic efficiency because only one memory unit is additionally required.
In recent years, a semiconductor memory, such as a flush memory, has been frequently used as a memory unit of a data storage apparatus because the semiconductor memory comprises no moving part and has high reliability. This type of memory unit comprises a plurality of memory chips, each of which includes semiconductor memories, and is configured to store data in the memory chips. However, in a memory unit comprising a plurality of memory chips, a memory chip may be damaged because of, for example, writing operations performed to excess.